The first crystal examined was the 1:1 complex between tick anticoagulant protein (TAP) and bovine pancreatic trypsin inhibitor (BPTI). TAP is a very potent factor Xa inhibitor from ticks, it has a negatively charged BPTI-like structure and it inhibits blood clot formation. The crystals are tetragonal and scatter to 1.5A resolution from conventional laboratory X-ray sources and to 1.0A resolution from the CHESS synchrotron source at room temperature. Although we have located the BPTI in the crystal structure, we have not been able to uncover the TAP molecule in electron density maps. By flash-freezing the crystals, we hope to obtain diffraction below 1.0A resolution and determine phases in collaboration with Herb Hauptman using the "shake and back" method. The experiment was disappointing because freezing affected the diffraction adversely. The second crystal was a ternary complex between factor VIIa (FVIIa)-tissue factor (TF) inhibited by another BPTI-like inhibitor (5L15). The orthorhombic crystals scatter to about 2.8A resolution at -150xC with two complexes per asymmetric unit (a=70, b=173, c=193 _). We hope to obtain 2.5 A or better data so that we can complete the refinement of the structure of the complex. The FVIIa-TF complex initiates the extrinsic pathway of the blood coagulation cascade. A 2.5A resolution data set was obtained which is presently being analyzed. The third crystal from which we would like to obtain diffraction data is a thrombin-heparin complex. Heparin is a therapeutic anticoagulant used regularly in clinical settings. We know how to freeze these hexagonal crystals that scatter to about 3.0A resolution. Increasing the resolution should permit the unambiguous location and refinement of the sulfated hexose units of the polysaccharide chain of the low molecular weight heparin used to form the binary complex crystals. These crystals did not survive the travel.